In the design of pressure switches, a rod slidable within a housing and movable by changes in external fluid pressure operates a plunger of a mechanically operated electrical switch. Resistance of the rod to movement, and hence the pressure at which the switch trips, is adjusted by a spring within the pressure switch that is biased against movement of the rod due to external fluid pressure. Adjustment of the compression on the spring changes the tripping pressure.
In one prior art pressure switch, model no. RS73 of Camrose Electric Controls Ltd. of Edmonton, Alberta, Canada, the spring is adjusted directly by a sleeve threaded onto the outside of the housing. As the sleeve moves along the outside of the housing, it compresses the spring by direct connection with the spring. This design, while it protects the inside of the housing from the environment, is prone to accidental movement of the sleeve, hence unintended changes in the tripping pressure.
In another prior art pressure switch, model DHTE of Barber Industries Ltd. of Edmonton, Alberta, Canada, the compression of the spring is adjusted by several wedges opposed to one another transverse to the direction of compression of the spring. As the wedges are moved towards each other by a screw abutting the end of one of the wedges, the longitudinal thickness of the combined wedges increases, thus compressing the spring. Again, with this device the adjustment mechanism is not enclosed and is susceptible to tampering.
The model DHTE of Barber also includes a mechanically operated pressure switch that is secured in place by a single screw, with the tripping point of the switch adjusted using a threaded piston rod inside a threaded trip plate. This trip point setting system is difficult to adjust.
The invention described and claimed here is intended to overcome the limitations of the known prior art. In one aspect of the invention, there is provided a pressure switch comprising a housing with an interior bore extending from one end to the other. A rod slidable within the bore and operated by external fluid pressure extends between a piston at one end of the housing and a mechanically operated electrical switch at the other end of the housing. A spring is disposed about the rod between a first stop on the rod and a second stop on the housing. The spring provides resistance against movement of the rod under external fluid pressure. The resistance of the spring to movement is adjusted by means disposed within the housing. A port in the housing allows access to the adjusting means and a cover is provided for the port. The cover preferably includes a ring disposed around the housing and threaded onto the housing over the port. The adjustment means preferably includes a sleeve engaging the spring, with the sleeve threaded into the bore of the housing for movement longitudinally within the housing. The sleeve includes plural radially extending slots disposed around the sleeve. Rotation of the sleeve changes the compression of the spring.
In a further aspect of the invention, the mechanically operated electrical switch is mounted on a switch mounting block by three cooperating screws. A central hold down screw passes through the switch mounting block and is threaded longitudinally into the housing. A pair of jacking screws flank the central screw and are threaded into the housing. Each of the jacking screws is received in a screw receiving counterbore in the switch mounting block to provide limit points for movement of the switch mounting block about the central hold down screw. Each of the screws is accessible from the second end of the housing, preferably through holes extending from the screw receiving counterbores.
The pressure switch may include a stop limiting movement of the cover away from the port, and may include a lock to prevent movement of the adjustment sleeve due for example to vibration of the pressure switch.
These and other aspects of the invention are described in the detailed description and claims that follow.